1. Field of the Invention
The present invention relates to an annular belt made of polyimide and a production method thereof. The present invention relates, in particular, to an annular belt made of polyimide which is useful as a belt for electrophotography such as a transfer belt and a fixing belt, and a production method thereof.
2. Description of the Related Art
Polyimide has generally been used in various fields since it has excellent heat resistance properties, anti-electrostatic properties, mechanical properties and the like. For example, a polyimide belt is used as a transfer belt for transferring a toner image to an image receiving medium (paper, cardboard, OHP sheet and so on) or a fixing belt for fixing a toner image to an image receiving medium in an electrophotographic image-forming apparatus such as a laser printer, an electrophotographic copying machine and the like.
The polyimide belt is mainly provided in a form of an annular belt. Since it is necessary to obtain uniform physical properties of the belt which exert influence on image qualities, the annular belt made of polyimide is generally produced by a so-called centrifugal molding method in which a cast method using a solvent is applied. In Japanese Patent Publication (KOKAI) No. 1993-77252, for example, it is proposed that a seamless belt is produced by using the centrifugal molding method in order to achieve an uniform resistance of an intermediate transfer belt. With respect to the annular belt made of polyimide for electrophotography, a sufficient improvement of the said belt cannot be achieved by methods other than the centrifugal molding method. Because a film applied to a mold is so soft until a solvent is evaporated that sagging of the applied film occurs and a dried film thickness becomes ununiform, the centrifugal molding method wherein a cylindrical mold rotates must be used in order to prevent such problems. When the film thickness becomes ununiform in a circumferential direction, nonuniformity of electrostatic properties occurs, and the image qualities are influenced by the nonuniformity.